1. Field of the Invention
This invention relates to the field of bandgap voltage references, and particularly to bandgap voltage regulators capable of providing an output voltage which is a multiple of the bandgap voltage.
2. Description of the Related Art
Voltage references based on the bandgap voltage of silicon and having low temperature coefficients are well-known. The Widlar bandgap voltage reference shown in FIG. 1a is one such circuit. When arranged as shown, the reference produces an output Vref given by:Vref=Vbe(Qc)+(Rb/Rc)ΔVbe,where ΔVbe is given by:ΔVbe=(kT/q)ln(Ja/Jb),where Ja and Jb are the current densities at the emitters of transistors Qa and Qb, respectively. When the circuit is arranged such that Vbe(Qc)+(Rb/Rc)ΔVbe=Vbg, where Vbg is the bandgap voltage for the fabrication process used to make the reference's bipolar transistors, the reference will be temperature compensated. The Vbe portion of Vref is referred to as the “CTAT” component, since Vbe is complementary-to-absolute-temperature (CTAT), and the ΔVbe portion of Vref is referred to as the “PTAT” component (proportional-to-absolute-temperature).
This design has several shortcomings, however. For example, the reference's operating current (I) is derived from the supply voltage (V+), and therefore varies with power supply variances. Vbe(Qc) must vary to tolerate the changing current, resulting in inaccuracies in Vref. In addition, if a reference voltage greater than the bandgap voltage is needed, an amplifier must be employed to multiply the bandgap voltage to the desired value.
One bandgap voltage regulator capable of producing a temperature compensated output voltage greater than Vbg is shown in FIG. 1b. P-n junctions are stacked as necessary to obtain a desired integral multiple of the bandgap voltage; here, transistors Qd, Qe, Qf, and Qg are stacked to provide the CTAT component of the output voltage. However, using this approach, a large multiple requires a large number of transistors, and only integral multiples of the bandgap voltage can be produced.